The present invention relates to an optical tape transport system.
Tape-based media are used for high-capacity data storage systems because the recording surface area on a reel of thin tape is very large relative to the cassette or cartridge volume. However, the potential for high-capacity storage can only be realized if the spacing and orientation between the recording surface and the transducer headxe2x80x94the head/media interfacexe2x80x94is held within very tight tolerances. For both magnetic and optical recording, these tolerances are much less than one micron. Furthermore, the stability of the head/media interface must be maintained under conditions of rapid tape transport so that the rate of data recording or readout will be high enough to match the data capacity.
In magnetic tape recording, the magnetic heads that sense and record magnetic domains must be held extremely close to the tape surface. A tape transport mechanism holds the tape in contact with the surface of the magnetic heads to maintain this tight spacing tolerance or axial stability. Although other parts of the tape path can be controlled using polished pins, rollers, or air bearing elements that handle the tape gently, the tape slides against the head elements with a high relative velocity, typically much greater than 1 m/s. This characteristic of magnetic tape transports gives rise to tape wear at the head/media interface.
Optical recording systems must also maintain a tight tolerance in the spacing between the media surface and the optical head. However, there is a relatively large spacing, typically 0.1-1 mm, between the media surface and the objective lens which is the closest element of the head. This spacing eliminates the risk of media wear at the head/media interface. On the other hand, the difference in spacing between heads and media precludes the use of magnetic tape transport methods for axial stabilization of optical tape.
In prior art optical recording systems, axial stabilization is provided by a closed-loop servo system that controls the position of the objective lens in response to optically-derived error signals. This approach is suitable for disk-based systems for which the media runout is periodic and limited to the lowest harmonics of the disk rotation frequency. Optical tape systems require additional stabilization to achieve adequate focus control. Axial flutter (normal to the tape surface) must typically be held below 0.5 xcexcm for frequencies above 1 kHz.
An optical tape drive is distinguished from an optical disk drive by other requirements. The optical tape transport must provide intermittent media transport while minimizing tape wear from frequent stops and starts. For linear data formats (data bands recorded down the length of the tape), bi-directional operation is required. For multitrack recording, the transport must provide simultaneous axial stability and media perpendicularity with respect to the optical axis across the field of view of the objective lens used for writing and reading.
It is an object of this invention to provide focus stabilization for optical tape that is written or read by an optical head while minimizing wear to the front and back surfaces of the tape during movement past the optical head.
It is a further object of this invention to prevent asperities on the surface of the tape from causing focus errors.
It is a further object of this invention to provide focus stabilization across the full width of the optical tape.
It is a further object of this invention to provide sufficient perpendicularity between the media surface and the optical axis across the width of a data band as it is written or read on the optical tape.
These objects are achieved by an optical tape transport system including an optical head, tape transport means, and a focus stabilizer element, the improvement comprising:
a) the stabilizer element including a cylindrical surface section supports the full width of the tape;
b) the stabilizer element being disposed on the opposite side of the tape from the optical head;
c) the optical axis of the optical head being perpendicular to the stabilizer surface at the point of intersection between the optical axis and the stabilizer surface;
d) the tape being supported at the stabilizer surface by an air film; and
e) the air film thickness at the point of intersection being in the range 1-50 xcexcm while the tape is transported for reading or writing.
The present invention has the advantage of reducing focus runout and improving focus stabilization for optical tape while it is written or read by one or more optical heads.
It is a further advantage of the present invention that it reduces wear to the surfaces of the optical tape medium caused by contact with tape drive components during transport past an optical head.
It is a further advantage of the present invention that it prevents asperities on the surface of the tape from causing focus errors.
It is a further advantage of the present invention that is provides focus stabilization across the full width of the optical tape.
It is a further advantage of the present invention that it permits a rigid interface between an optical head and an associated tape transport, improving alignment tolerance between the head and media.